This invention relates to digital-to-analog and analog-to-digital converters and more particularly to such converters which operate according to the .mu.255 compression law.
For a number of years, voice signals have been transmitted along telephone lines by pulse code modulation (PCM). In PCM, each sample of an analog signal is quantized, that is, it is rounded off to one of a number of predetermined quantum levels, the difference between levels being quantum steps. Each quantum level is associated with a digital code word of several binary bits. Thus, a continuing analog signal may be represented by a series of multiple bit code words.
Because each sample is rounded off to a quantum level, an error in the coded signal known as the quantizing error appears as quantization noise. If the spacing between quantum levels is uniform, that is where the quantum steps are constant, the quantization error will be within a set range independent of the level of the signal; thus, the signal to noise ratio will vary with the level of the signal. A constant step or linear code providing the minimum acceptable signal to noise ratio can be provided by decreasing the spacing between quantum levels; however, this results in less spacing than is required to meet the minimum signal to noise ratio with the higher level signal. Decreased spacing between quantum levels requires an increased number of bits in each code word and it is preferred that the number of bits be minimized.
In order to provide a substantially uniform signal to noise ratio over the entire range of analog signal amplitudes, and thus decrease the number of bits per word while maintaining a minimum signal to noise ratio, nonlinear digital codes have been developed. In these codes, the steps between quantum levels increase at higher quantum levels. Thus the signal error increases with the level of the signal and the ratio of signal to noise remains substantially constant. One such code follows what is known as the .mu. compression law. In my prior U.S. Pat. No. 3,882,484 I provided digital-to-analog and analog-to-digital converters which could operate in accordance with the .mu. compression law. In that patent, in the digital-to-analog converter, logic circuitry controlled a step attenuator in the feedback circuit of an op-amp. In the analog-to-digital converter the analog signal was applied through a step attenuator to a comparator, and logic circuitry responsive to the comparator output controlled a successive approximation determination of the digital signal.
Although my prior patent discloses a successful encoder/decoder for coding according to the .mu. compression law, it does not provide coding and decoding of the more recent .mu.255 compression law coding. In that code the analog signal is quantized linearly within segments or groups of steps, that is, the space between quantum levels within each segment remains constant. However, each segment, which includes sixteen steps, is of a different length; the segment lengths and thus the spacing of steps within the segments increases with the signal input. As shown in FIG. 1, the segments between end points 22 approximate a .mu. law coding while steps within each segment appear as a simplified linear coding.
Referring to FIG. 2, the .mu.255 code word includes a sign bit 24, three segment bits 26, and four step bits 28. The three segment bits define one of eight segments for a plus or minus signal. The quantization levels at the end points of each segment vary nonlinearly with the analog signal. The four step bits define any of sixteen steps within the segment defined by the segment bits. The steps within the segment vary linearly with the four-bit step code; that is, the space between any quantum level within the segment and one of the end points of that segment can be determined by multiplying the value of the four-bit step code word by a first constant and adding a second constant which may be zero.
For a more specific definition of the .mu.255 compression law with respect to both coding and decoding reference is made to the CCITT recommendations, Vol. III, recommendations 6.711 at pages 375 and 376. It should be noted that, in encoding, the first segment includes a first interval or step of one unit whereas all others of that segment are of two units. This is because the digital code defines the value between two encoder decision values. By providing a half-step in the encoding process, the value of the first digital code will be at the first quantum level and so on.
Although a number of methods have been developed to realize direct coding and decoding of the .mu.255 compression law, most of these methods are complex and require a number of op-amps and high precision analog-to-digital converters.
An object of this invention is to provide circuits for the analog-to-digital and digital-to-analog conversion of signals in accordance with a compression law which results in nonlinearly weighted groups, each group including linearly weighted steps.
It is a more particular object of this invention to provide analog-to-digital and digital-to-analog converters which operate according to the .mu.255 compression law.
It is a further object of this invention to provide such converters which are inexpensive and are of relatively simple design yet which provide an accurate conversion as set forth by the CCITT recommendations.